General problems relating to the basic understanding of CdS/Cu2S solar cell operation, to material aspects of the cell and to manufacturing methods and cell engineering are discussed

Boer, K. W.

English

NASA Technical Reports Server

ASSESSMENT OF THE INTERNATIONAL WORKSHOP ON 
CdS SOLAR CELLS 
Karl W. Bger 
Institute of Energy Conversion 
University of Delaware 
Newark, Delaware 197 11 
The National Science Foundation sponsored an International Workshop on 
CdS/CuzS solar cells and other abrupt heterojunctions held at  the University of 
Delaware. Approximately 100 participants, including representatives of NSF, 
ERDA, NASA and EPRI, discussed during the three day workshop (April 30- 
May 2) critical questions relating to the performance, stability and economics 
of these cells with the aim to assess the current state of the a r t  and future 
potential. 
In three parallel sessions, general problems relating to the basic under- 
standing of the cell operation, to material aspects of the zell and to manufac- 
turing methods and cell engineering were discussed. 
In a general meeting an the last day of the workshop, a synthr+s was 
attempted in extensive discussions and summarized at  a meeting of the Chair- 
men and Associate Chairmen after the formal adjournment of the workshop. 
Here It was agreed that the following statement summarized the principal out- 
come of the meeting. 
points represent the principal areas of agreement. 
To the extent that there was a definable consensus, these 
Initial Performance and Possible Limits 
Peak performances of 7-8.570 conversion efficiency have been demon- 
strated at room temperature. Peak values of short circuit currents of 25 
miimp. cmd, open circuit voltages of 0.53 volts and fill factors of 7570 have 
h-zn fl'monstrated. Most investigations show that the highest short circuit 
. nts a re  obtained with close to stoichiometric CqS.  
The highest achievable short circuit current under 100 mwatts/cmz 
-?ural insolation is about 35 mamps/cm2 for such cells, allowing 2070 optical 
Losses. The tieoretical limit for the f i l l  factor is agreed to be about 8070. 
Two major approaches to increasing the open circuit voltage were agreed 
to; namely, recombination at  interface states and modifying the band inter- 
connect. A reasonable goal of 0 .65  volts was projected and a theoretical upper 
limit of 0.86 volts was preeented. 
On the basis of the above individual parameters overall conversion 
efficiencies in excess of 1570 are  implied. 
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Cell Reliability 
It was generally agreed that for  terrestr ia l  applications, degradation is  
not an obstacle to utilization of the CdS cell and lifetimes in excess of 20 
years  a r e  achievable. A major contribution to observed degradation in the 
past was the result  of inadequate protection from atmospheric constitutents. 
A major degradation mechanism is related to stoichiometry changes in the 
CuzS layer. 
Economic Factors 
There a r e  a t  least two production techniques capable of producing 
economic CdS solar cells. 
and 10 cents/peak watt rzsing the techniques of sensitivity analysis. 
achieve such projected costs a sufficient production yield of high efficiency 
cells is necessary. There a r e  indications that such yields a r e  achievable. 
Projected costs/watt were stated to be between I 
In order  to 
Gene1 11 Recommendations 
It is recognized by the members of the workshop that the complexity of 
the heterojunction solar cells warrants a concerted research program with a? 
over-critical mass  to systematically approach and solve the key problems to 
fur the r improvements 
Such improvements shall include an increase of conversion efficiency 
above 1070, a decrease in thickness of the CdS layer below 10pm, the develop- 
ment G. an inexpensive grid and an inexpensive hermetic encapsulation. 
must also include the achievement of a production yield in excess of 90% for 
high efficiency cells within one percentage point deviation. 
expectancy in excess of 20 years  under actual operating condit'.ons is desirable. 
It 
Finally, a life 
The current state of the art shows feasibility of achieving these goals. 
A great variety of celLs have been fabricated using different techniques, elec- 
trodes and encapsulations and have demonstrated that cell efficisncies in excess 
of 570 can bc achieved in various ways. 
In order to accelerate further progress  a systematic research program 
is recommended: 
1. Substrates: A wide variety oi substrates is currently used with 
success including glass, Mo, NiFe, Copper and Kapton with 
silver pyre. The parameters  of influence to cell performance 
shall be identified and substrate8 shall Le selected which a r e  
conducive to high efficient ard economic cells. 
40 1 
2. Base Electrodes: Currently predominantly zinc is used but other 
materials such a s  A l ,  Pd-Ag-Ti and conducting glasses a r e  known 
to produce ohmic contacts to CdS. Other materials and deposition 
methods shall I js identified as ohmic contacts of stfficient conduc- 
tance and, -for back wall cells - of sufficient optical transparenc). 
3. CdS Layer: It is h o w  that CdS of different deposition and structure 
yields good CdF ,;ar ceils. Critical parameters shali be identi- 
fied relating t Lructurc, stoichiometry, doping and surface 
morphology to obtain high efficiency arld stable cells. 
methods of deposition and treatments shall be investigated to 
economically produce such layers. 
Alternative 
4. CuxS L,ayer: Evidence is obtained that Chalcocite with X > 1.995 
deposited in a variety of w a y s  yields cells with high colle&on 
efficiency. 
shall be iderbified with critical parameters relatirbg to structure 
and doping to obtain highest collection efficiencies. 
surface passivation and to stabilize the electrical parameters of 
the CuzS layer shall be investigated a s  well as alternative means 
of deposition and post-deqosition treatments. 
:'he range of permissible deviation from stoichiometry 
Means for 
5. Top Electrode: Materials and means of attachment shall be 
identified to achieve stable ohmic contacts ta Cu2S which could 
replace the currently used gold. 
Low cost electrodes shall be developed with big', collection efficiency 
(for front wal l  cells wj th high optical transparency). 
6. E.xa sulation: Materials or  mat t  r ia l  combinations shall be idcnti- 
*n combine antireflection coating with a hermetic 
sealing and possibly surface passivation. Inexpensive encapsulation 
techniques shall be developed. 
7. Arrays: Critical parameters shall be identified for automatic a r ray  
fabrication. 
a r r ays  shall be developed. 
Means to inexpensively produce and encapsulate such 
8. . ldernative Heterojunctions: The investigation of alternative 
material  combinations shalt be continued and their potential shall 
be identified in comparison with the CdS/CuZ solar -cell in respect 
to maximum possible efficiency, ease of fabrication, degradation 
mechanism and other relevant parameters influencing reliability 
and economics. 
9.  Cell 0 eration: Efforts shall be continued to improve the under- *stan ing o e physics of the cell operation. 
10. Diagnostic Tools: Diagnostic tools shall be further developed to 
identify all essential parameters of the cell and to control repro- 
ducible manufacturing of all components of the cell. 
Meaningful accelerated life testi; shall be developed 
developed cells shall be promptly submitted to such 
tests. 
12. Documentation and Exchange of Information: All  Iesults shall be 
recorded with attention to ease of access  and data retrieval. Close 
relation of the different parts of the program anrl swift exchange of 
information is empkasizt .le 
In addition, systems analyses of the relation between initial cell costs, 
performance and useable lifetime shall bta pursued to give guidance to cell 
research and development. 
This workshop has served a useful function tc. assess  the current state of 
the art and to provide guidance for future reseimch. It is pioposed to hold 
such workshops periodically. 
40 3 


Assessment of the international workshop on CdS solar cells

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